Fluorenones are compounds of high utility as intermediates for organic synthesis or raw materials for resins. Known examples of the preparation process of fluorenones include oxidation of fluorenes and liquid phase oxidation of benzophenone using palladium acetate as an oxidizing reagent.
As the process for the oxidation of fluorenes, liquid phase reaction is disclosed in German Offenlegungsschrift DE-1,262,268 and 1,940,051, while vapor phase reaction is disclosed in JP-A-60-233028 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
The raw material fluorenes are however mainly obtained by separation from tar. From the viewpoints of the separation cost and problem of limited resources, use of the tar is not economically advantageous.
Concerning a process for obtaining fluorenes by synthesis, catalytic cyclodehydrogenation of an alkylbiphenyl compound is described in U.S. Pat. No. 3,325,551. According to its description, 9,9-dimethylfluorene is prepared from 2-isopropylbiphenyl as shown specifically by the following reaction formula: ##STR1##
The above process is however economically disadvantageous because the raw material biphenyl compound should be separated from a tar fraction similar to the case of fluorenes or a coupling reaction of benzene at high temperature is required where obtaining by synthesis. In addition, the raw material alkylbiphenyl compound requires introduction of an alkyl group into a specific position of a benzene ring, so that such a compound is not an easily available compound.
On the other hand, processes for the synthesis of a fluorenone from benzophenone are disclosed in J. Org. Chem. Vol. 40, No. 9, 1365-1367(1975); J. Chem. Soc. Perkin Trans Vol.I. 11, 1236-(1976); and the like.
The above processes are however accompanied with the industrial problems such as necessity of a large amount of palladium, low productivity per expensive palladium, long reaction time and insufficient yield, because the reaction effected in these processes is a reagent reaction in which palladium acetate is added to benzophenone in at least an equivalent amount, and is also a liquid phase reaction.
Accordingly, in view of the above circumstances, an object of the present invention is to provide an industrially advantageous process for the preparation of fluorenes at a low cost and in a high yield.